CFB reactors or combustors used in the production of steam for industrial process requirements and/or electric power generation are well known in the art. FIGS. 1, 2, and 3 illustrate various known CFB designs. A CFB reactor or combustor, generally referred to as 1, is shown therein. Fuel 2 and sorbent 4 are supplied to a bottom portion of a reactor enclosure or furnace 6 contained within enclosure walls 8, which are normally fluid cooled tubes. Air 10 for combustion and fluidization is provided to a windbox 12 and enters the furnace 6 through apertures in a distribution plate 14. Flue gas containing entrained particles or solids 16 (reacting and non-reacting particles) flows upwardly through the furnace 6, releasing heat to the enclosure walls 8. In most designs, additional air is supplied to the furnace 6 via overfire air supply ducts 18. A bed drain purge 19 is also provided.
Both reacting and non-reacting solids are entrained in the flue gas within the furnace 6, and the upward gas flow carries these solids to an exit at an upper portion of the furnace 6. There, a portion of the solids are collected by a primary particle separator 20 and returned to a bottom portion of the furnace 6 at a controlled or non-controlled flow rate. The collection efficiency of the primary particle separator 20 is commonly not sufficient for the retention of particles in the furnace 6, as required for efficient performance and/or for the required reduction of the solids content in gases discharged to the atmosphere. For this reason additional particle separators are installed downstream of the primary particle separator 20.
Referring to FIG. 1, in one known CFB reactor arrangement a secondary particle separator 22 and its attendant solids recirculation means 24 are installed to collect and recycle particles passing the primary particle separator 20 as needed for efficient CFB operation. The gases and solids release heat to convection heating surfaces 26 located between the primary and secondary particle separators 20, 22, respectively. A final or tertiary particle separator 28 is provided downstream (with respect to the flow of flue gas and entrained particles 16) of the secondary particle separator 22 for final gas cleaning to meet particulate emission requirements. A purge system 30 may be employed to purge solids collected from the flue gas by the secondary particle separator 22.
In another arrangement, shown schematically in FIG. 2, the secondary particle separator 22 is the final particle separator. In this case, to improve the particle retention as needed for efficient CFB furnace 6 performance, the solids or particles collected by the secondary particle separator 22 may be partially recirculated through the recycle transport line 24 to a lower portion of the CFB reactor 6. A purge system 30 purges solids collected from the flue gas by the secondary particle separator 22.
When solids recirculation from the secondary particle separator 22 is needed for efficient unit operation, the rate of recirculation corresponds to the CFB system material balance with a given solids input flow and is a function of the physical characteristics of the solids and efficiencies of the primary and secondary particle separators 20, 22 respectively, and limits or targets imposed on the recirculation rate by one of the following: a) the capacity of the solids recirculation means 24; b) the maximum acceptable solids loading through the convection heating surface 26 downstream of the primary particle separator 20; c) the flow rate that provides the optimum CFB reactor performance (in terms of combustion efficiency, sorbent utilization, convection surface erosion, operating and/or maintenance cost of the solids recirculation system) and d) the low limit of the bed temperature in the CFB furnace 6.
When the solids recirculation rate from the secondary particle separator 22 is restricted as compared to that rate which would otherwise be obtained as determined by the material balance due to one of the limits described above, the excess of circulating solids is removed from the secondary particle separator 22 for disposal through the purge system 30, shown in FIGS. 1 and 2, to accommodate the recirculation rate limitation.
In known systems a minimal solids inventory is maintained in a secondary particle separator hopper 32 by controlling the purge rate through purge system 30. In these systems, an increase in the flow rate of solids recirculated from the secondary particle separator 22 to increase the solids inventory in the CFB reactor 1 can only be done slowly. The rate of the recirculated flow (and inventory) increase is dictated by the change of the secondary particle collector purge flow rate, which is reduced to zero when the recirculation flow starts to increase. In FIG. 1 systems, this purge flow rate is typically not more than 10% of the recirculation flow, and the rate of recirculation flow increase is insufficient for responsive reactor inventory control.
FIG. 3 schematically shows a known CFB reactor or boiler system of the type disclosed in U.S. Pat. No. 4,538,549 to Stromberg. In this system, the bed temperature in the CFB reactor furnace 6 is controlled by changing the inventory of circulating solids in the furnace 6 by regulating the circulation rate of solids collected by the primary particle separator 20 and stored in a primary particle storage hopper 34 placed underneath the primary particle separator 20. The mass of solids in primary particle storage hopper 34 is varied depending on CFB reactor control requirements. When more inventory is needed in the furnace 6 to reduce the bed temperature, the solids circulation rate through a standpipe and non-mechanical L-valve 36 connecting the primary particle storage hopper 34 with the bottom portion of the reactor enclosure or furnace 6, is increased. A part of the stored bed material is thus transferred to and becomes part of the furnace 6 inventory. When the CFB reactor inventory is to be decreased, the opposite action takes place which results in solids accumulating in the primary particle storage hopper 34.
In the CFB system shown in FIG. 3, the flow rate of solids recirculated from the secondary particle separator 22, is "uncontrolled but selfadjusting" (per Col. 7, lines 16-19 of U.S. Pat. No. 4,538,549) as determined by the material balance. However, operational experience with the CFB system reactor or boiler and control method of U.S. Pat. No. 4,538,549 has shown the following shortcomings:
a) transport of solids stored in the primary particle storage hopper 34 in the packed bed regime causes flowability problems due to the tendency of the particles in a packed bed to agglomerate at temperatures of about 1600.degree. F., which is typical for fluidized bed combustion applications; and PA0 b) the hot particle storage, transfer, and control means required to accomplish this control method represent a considerable cost and contribute to the complexity of the CFB design.
An improved CFB reactor has been suggested (U.S. patent application Ser. No. 08/037,986, filed Mar. 25, 1993, assigned to The Babcock & Wilcox Company) in which solids are collected by an entirely internal primary particle separator which also returns particles collected thereby internally and directly to a bottom portion of the CFB reactor. This improved CFB reactor thus eliminates the need for any external recirculation means such as standpipes and L-valves, which considerably simplifies the CFB reactor arrangement and reduces its cost. A disadvantage of this concept as compared with that of U.S. Pat. No. 4,538,549 is that it does not provide for control of the bed temperature by controlling inventory of the circulating material in a CFB reactor via regulating the solids recirculation rate from the primary separator.
It is thus apparent that a need exists for a method and apparatus for controlling a bed temperature in a CFB reactor or combustor that does not rely upon controlled recirculation of particles collected by a primary particle separator.